How we change what others think, feel, believe and do
Convince the person that you already know what you are asking of them.
Show them how much you know of their background and activities. Indicate that there is much more that you also know. Let them think you are all-knowing and all-seeing.
Make a big show of how you discovered this. For example, tell them that you have already been told this by somebody else, perhaps an accomplice of the person being interrogated.
Watch closely their response to this news. They will react slightly differently if you are telling the truth or otherwise.
Michael, I've got to tell you that Sidney has just confessed and told us all about how you pulled the trigger. It was you, wasn't it?
Listen, Sarah, I know you bunked off school today. You were seen, so there's no point trying to hide it.
I know about the raid. I know about all the planning. You know that. And I know who was there. Now I'm going to tell you what you did. Or do you want to tell me first?
When the person believes that you already know what they are trying to conceal, the act of confession goes from one of betrayal, perhaps of others or one's principles, to a simple release of pent-up tension.
A reversal of this is to tell them something that you know did not happen and watch their reaction. This will at least help you calibrate their response to untruth. It may also goad them into telling you what actually did happen.